refactor(pox-4-tests): Rename Wallet 'client' to 'stackingClient'

Improves code clarity by renaming the 'client' field in the Wallet
type to 'stackingClient'. This change better specifies the purpose of
the client as specifically for stacking operations within the PoX-4
testing framework. The update is applied across all instances where
the field is referenced, ensuring consistency and enhancing
readability.

contrib/core-contract-tests/tests/pox-4/pox-4.stateful-prop.test.ts

@@ -60,7 +60,7 @@ describe("PoX-4 invariant tests", () => {
         btcAddress,
         signerPrvKey,
         signerPubKey,
-        client: new StackingClient(stxAddress, devnet),
+        stackingClient: new StackingClient(stxAddress, devnet),
         ustxBalance: 100_000_000_000_000,
         isStacking: false,
         hasDelegated: false,

contrib/core-contract-tests/tests/pox-4/pox_CommandModel.ts

@@ -21,7 +21,7 @@ export type Wallet = {
   btcAddress: string;
   signerPrvKey: StacksPrivateKey;
   signerPubKey: string;
-  client: StackingClient;
+  stackingClient: StackingClient;
   ustxBalance: number;
   isStacking: boolean;
   hasDelegated: boolean;

contrib/core-contract-tests/tests/pox-4/pox_StackStxCommand.ts

@@ -62,7 +62,7 @@ export class StackStxCommand implements PoxCommand {
     // generated number passed to the constructor of this class.
     const maxAmount = model.stackingMinimum * this.margin;
 
-    const signerSig = this.wallet.client.signPoxSignature({
+    const signerSig = this.wallet.stackingClient.signPoxSignature({
       // The signer key being authorized.
       signerPrivateKey: this.wallet.signerPrvKey,
       // The reward cycle for which the authorization is valid.